Check the Match Option set at PO Shipment Level.
If Match Option is set to PO then while entering the Invoice Matching Action should be selected as Purchase Order. Only then Purchase Order Match Form will open.
If Matching Action is set as Receipt the Receipt Match Form will open and since Match Option is PO, such Purchase Order can not be queried through Receipt Match Find window.
Since Match Option is set to PO at the purchase order the Matching Option at invoice must be selected as Purchase Order then open the Match window which will open Purchase Order Match form instead of Receipt Match form. Provide the PO Number and query the PO.
